1. Field of the Invention
The present invention relates to an ink jet recording apparatus that records by discharging liquid ink.
2. Related Background Art
Conventionally, the ink jet recording apparatus that records by discharging liquid ink has been known.
There are roughly two kinds of ink jet recording apparatuses. One is a recording apparatus of serial scanning type which is provided with a carriage having an ink jet head mounted thereon that scans to perform recording. The other is a recording apparatus of line type having a line type ink jet head fixed therein, which is provided with discharge nozzles to discharge liquid ink for recording on the width portion of a recording sheet while the recording sheet is carried immediately below the head.
For the serial scanning type recording apparatus, there has been a tendency in recent years that the length of the nozzle array is made as long as 0.5 to 1 inch so as to reduce the frequency of carriage scanning per recording sheet for faster recording.
Here, an ink jet head is almost made practicable with 300 to 600 nozzles which are arranged at a pitch of {fraction (1/600)} inch or the like.
An apparatus of such kind comprises a printer controller that develops image data received from a host computer or the like into bitmap data, and converts them into recordable information; an engine control unit that controls each of functional units in the interior of a recording apparatus in accordance with commands or the like received from the printer controller; and each of the functional units that actuates each inner function of the recording apparatus.
Also, for the serial type recording apparatus that represents the ink jet recording apparatus, the bitmap data are developed per raster area at the time of output. Then the data are transmitted for the execution of recording.
Further, there has been widely in use the so-called flat-bed type scanner which reads images from a source document set on a source document stand. Typically, it has been used in such a way that the host computer fetches images thus read out and processes or edits them for outputting them to other recording apparatus or the host computer reads a preformatted sheet such as represented by a voucher, and after an input of numeral data or the like, it outputs them on the correct positions on such preformatted sheet.
For the conventional art described above, however, there are problems encountered as follows:
(1) As the serial scanning type recording apparatus forms images while repeating its carriage scans, there inevitably occur seams in the direction at right angles to the carrying direction of a sheet.
For example, when vertically ruled lines are recorded, these lines are deviated to the left and right at each seam, and the lines are disconnected to form serrations in some cases.
There are two causes for this occurrence. One is difference in carriage speeds for the first scanning and the scanning to follow, that is, uneven speed thereof, which may cause deviated impact positions of ink discharged onto a recording sheet. The other is brought about by the nozzle array, which is not fixed in the carriage correctly at right angles to the direction of the carriage scanning, or by the carriage itself, which may be inclined when the carriage scans. FIG. 15 shows the state where the ruled lines are diagonally connected in continuation.
Here, actually, the former example is rare. Even if the uneven speeds occur, there is almost no case that the unevenness in the first scanning speed is allowed to change in the one that follows. On the other hand, the latter examples are often encountered along with the tendency that nozzles are made in increasing numbers.
The angle of the head fixed in the interior of the carriage tends to be deviated from the correct position due to variations in the shape of the carriage or variations in the shape of the head. One deviation appears as an inclination of the nozzle array. The more nozzles that are multiply arranged, the larger the length L becomes in FIG. 15. A slight deviation of angle may cause a greater degree of serration when vertically ruled lines are recorded, and the appearance becomes unpresentable.
Also, when color recording is executed, color tones may change depending on the amount of deviation in the impact positions of one color ink and the other color ink. Therefore, when heads are arranged side by side, the amount of deviation in the impact positions is caused to differ on one edge side of a nozzle array and the other edge side thereof if the installation angle is different for each of the heads. Thus, the color tones may change to make the resultant images uneven in colors carried by them as a whole.
(2) As described earlier, the serial scanning type recording apparatus forms images by repeating the carriage scanning and recording sheet feeding. Thus, if the precision is not good enough for feeding the recording sheet, images are affected. In other words, if the feeding is shorter than the length of the nozzle array, images are overlapped to present higher density which becomes black lines. If, on the contrary, the feeding becomes longer, white lines are created inevitably.
In order to prevent them, there has been known the so-called multi-pass recording in which images are formed by feeding the recording sheet in an amount {fraction (1/2)} or {fraction (1/3)} of the length of the nozzle array. In this case, the length of the nozzles cannot be utilized fully, which obviously leads to a problem that throughput becomes lower.
Also, in order to suppress the degradation of images recorded by one pass to the extent that the human eye cannot discriminate it, there is a need for securing the precision of recording sheet feeding within an error of approximately ±15 μm. Also, the fluctuation of the sheet feed roller should be minimized; a special coating process should be given to the surface of the sheet feed roller so that the coefficient of the surface friction coefficient is made greater; and a motor having high rotational precision should be adopted or the power transmission gears should be made highly precise, among some others. These requirements result in a problem that the costs of parts become higher inevitably in every aspect.
(3) For the serial scanning type recording apparatus, the carriage scanning should be repeated many times in order to draw the vertically ruled lines as shown in FIG. 15. Here, the carriage reaches a constant speed after its acceleration period, thus eliminating the instability of speed for making highly precise impacts of ink on a recording medium. Ink is discharged at this constant speed. Then, the movement of the carriage is controlled to be suspended after its deceleration period. After that, the movement is reversed. As a result, the more frequent the scanning, the greater becomes the total of such deceleration period of time. The resultant recording time is made longer per page eventually.
Also, when color recording is performed by the heads arranged side by side, there is a need for adding the scanning distance equivalent to the total pitches of head arrays to the scanning distance needed for the black monochromatic recording. Therefore, the larger the pitches, the greater becomes the total scanning distance per page, and the recording time becomes longer accordingly.
(4) For the line type recording apparatus, no problem exists that corresponds to the aforesaid problems (1) to (3). However, if, for example, an apparatus that carries an A-4 sized sheet vertically should be used, it becomes necessary to arrange approximately 5,000 nozzles at a pitch of {fraction (1/600)} inch. As a result, the cost of the ink jet head becomes higher in consideration of the anticipated unfavorable yield of manufacture. Particularly in the case of a color recording apparatus, the heads of plural colors, such as black, cyan, magenta, and yellow, should be arranged in parallel, and the costs are made higher still.
Also, for the head which should be made larger, the capping mechanism to prevent nozzles from being dried and the ink suction mechanism to recover the clogged nozzles are made larger accordingly, hence making it difficult to reduce the costs of manufacture and make the apparatus smaller as a whole.
(5) As a common problem that exists both for the serial type and line type recording apparatuses, there is a restriction imposed upon the usable kind of recording medium.
For the serial type recording apparatus, the area of the recording medium holding unit (the so-called platen) that faces the orifice surface of an ink jet head is rather limited. Therefore, a part of recording medium should be pinched by a pair of rollers or by a roller and a spur when recording is performed. Usually, the passage of the recording medium, which is formed by these rollers or guides, is not exactly on a straight line, but on a curved or zigzag line. As a result, a recording medium whose thickness is 1 mm or more, such as a CD-ROM or a piece of extremely hard cardboard, cannot pass such passage, thus making the execution of recording impossible.
In this respect, there are some apparatuses which are provided with a manual inlet port to make those recording mediums recordable, but drawbacks still exist such as to cause a recording medium to be folded or curled. Also, a recording medium whose surface is made glossy has a lower coefficient of friction with the rollers. The rollers tend to rotate idly so that the recording medium is not carried or the precision is extremely lowered even if it can be carried. These may bring about a problem that the quality of recorded images is degraded.
(6) If images should be processed before output as described earlier, there is a need for the provision of a scanner and a recording apparatus in addition to a host computer. Here, the problem is that this requires a larger installation area. Also, a flat-bed type scanner is not made to carry source documents automatically, which necessitates the user to place them one by one on the scanner. Therefore, it presents a problem that the operability becomes unfavorable when many sheets of source documents should be read.